Manual

Model 45A User Guide Issue 3, August 2015
Studio Technologies, Inc. Page 21
indication that an active intercom circuit
is connected. In this case the 200 ohm
termination is expected to be provided
elsewhere, typically as part of the external
power supply, and the Model 45A’s termi-
nation is “lifted.”
When the Model 45A is Serving as the
2-Wire Intercom Power Source
When configuration DIP switch 2 is set
so that the Model 45A provides 2-wire
(PL) power its interface supplies 30 volts
DC on pin 2 of the 2-wire (PL) intercom
connectors. A maximum current draw of
300 milliamperes is available. This current
is sufficient to power various intercom
user devices such as small user stations
and beltpacks. A common broadcast
application would be to use RTS BP-325
beltpacks. Select the connected devices
so that their total current doesn’t exceed
300 milliamperes. That’s not always the
easiest figure to calculate but a web
search will generally find specifications for
all commonly used devices. For example,
a search finds that the original version of
the BP-325 consumes a maximum of 85
milliamperes of current. According to this
figure up to three of these units can be
connected to each Model 45A Interface.
Newer versions of the BP-325 use surface
mount component technology and have
a lower maximum current draw of 65 mil-
liamperes. Four of these “modern” BP-325
units can easily be supported.
The Model 45A’s intercom power supply
circuit operates under software control.
This allows detection of fault conditions
and protection of both the Model 45A’s
circuitry and connected intercom user
devices. Upon initial Model 45A 2-wire
intercom power up no monitoring of the
intercom power output takes place for
a period of five seconds. This allows the
Model 45A’s circuitry and the connected
intercom user devices to stabilize. The
LED associated with pin 2 will light to
indicate that the output is active. After this
initial 5-second period monitoring be-
comes active. A fault condition is detected
if the power on pin 2 falls below 24 volts
for a continuous 1-second interval. The
hardware and software responds to this
condition by turning off the power source
to pin 2 and flashing the pin 2 LED as
a warning. After a 5-second “cool-down”
interval the output returns to the same
condition as upon initial power up. Power
is again applied to pin 2, the pin 2 status
LED will light, and monitoring won’t begin
for another five seconds. A full short-
circuit condition applied to the Model
45A’s 2-wire connectors will result in a
continuous cycle of six seconds on (five
seconds for start up and one second for
detection) and five seconds off.
As a diagnostic aid the 2-wire (PL) power
status LED associated with pin 3 always
remains active, even when the Model 45A
is supplying power to the 2-wire party-line
intercom circuit. Whenever DC in excess
of approximately 20 volts is present on
pin 3 the LED will light. This condition will
typically not exist but could prove useful
in special circumstances.
2-Wire Circuit Serves as the
Unit’s Power Source
If no external source of 24 volts DC is con-
nected and configuration DIP switch 1 is in
its on (up) position, the connected 2-wire
party-line circuit can provide power for the
Model 45A. If this condition is active the
2-wire (PL) input power LED will light. The
2-wire (PL) power status LED will also be